In electric ranges, it has been common to connect a plurality of top surface heating element circuits to a source of AC power wherein each circuit comprises a relay in series between the source and a heating element. In response to the operator turning a control knob, a conventional surface heating element controller activates the appropriate relay to energize the selected heating element. Of course, a plurality of heating elements can be energized simultaneously.
A microprocessor-controlled electric range was developed. Added to each of the surface heating element circuits was a transistor which functioned as a switch to activate the relay. The switching function of each transistor was controlled by the microprocessor. More specifically, the base of each transistor was connected to the microprocessor and, in response to an operator input command to the microprocessor, it would bias the base of the appropriate transistor to render it conductive. As a result, -24 volts DC on the emitter side of the transistor was applied across the corresponding relay to ground. Accordingly, the relay was activated thereby closing the normally open contacts to energize the heating element. It was realized that if one of the transistors failed in a mode whereby it became a short circuit, the relay selectively associated with it would be inadvertently activated thereby energizing its respective heating element. In order to eliminate this potentially dangerous failure mode, a common transistor was put in series between each of the selectively associated transistors and the -24 volts DC. This common transistor was also controlled by the microprocessor and was switched to a conductive state by the microprocessor simultaneous to any one or more of the selectively associated transistors being closed. Accordingly, if one of the selectively associated transistors failed to a short circuit, its associated relay would not be activated because the common transistor still provided an open circuit between the -24 volts DC and ground.